The human spine is comprised of a plurality of components (e.g. vertebral bodies, intervertebral discs, posterior bony structures) which collectively protect the spinal cord enable the normal physiological motions of flexion (bending forward), extension (bending backwards), lateral bending (bending side to side), and rotation (twisting). These normal physiologic motions may be impeded and/or pain generating when any of a number of conditions exists, including but not limited to disc degeneration, trauma, and deformity (e.g. scoliosis). Depending upon the condition, surgical intervention may be required to restore the normal physiologic function of the spine at the affected region. One form of surgical intervention involves fusing one or more levels within the spine. This is typically accomplished by performing a discectomy (removing part or all of an intervertebral disc), introducing a height-restoring implant into the disc space, and then immobilizing the adjacent vertebral bodies on either side of the intervertebral implant such that a bony bridge may form between the adjacent vertebral bodies to fuse that particular spinal segment. The step of immobilizing the vertebral bodies may be accomplished in many ways, including the use of pedicle screws (fixed axis or multi-axial) and rigid rods, wherein the pedicle screws are introduced into the pedicles associated with the respective vertebral bodies and the rigid rods are locked to each pedicle screw to prevent motion between the adjacent vertebral bodies.
In the lower lumbar spine, as well as spinal deformity at any level in the thoracolumbar spine, pedicle location in the coronal and axial planes varies between levels of vertebra. This creates significant difficulties in spinal instrumentation assembly. Several methods have been utilized to address these difficulties. First, surgeons bend the rods in the coronal plane. However, because the rods are simultaneously bent to create lordosis, this method tends to weaken the rods. Also, the bend is inconsistent and could not be used for minimally invasive surgery. Second, surgeons have tried rotating the rods into the coronal plane. However, this orients the curve intended for lordosis into the coronal plane, thus losing its lordotic shape and lowering the effectiveness of the spinal stabilization system. Third, surgeons have compromised the screw insertion path into the pedicle in order to facilitate assembly of the spinal stabilization system. Unfortunately, inserting the screws in a less than optimal position results in a less than optimal fixation and increases the probability the screw will loosen or even separate from the pedicle. Forth, surgeons have tried skipping levels to facilitate assembly. However, this method potentially compromises the stability of the spinal stabilization system and may increase the probability of rod breakage. Lastly, there are spinal fixation systems that are connector-based. However, these systems have various degrees of difficulty. For instance, the connector tends to rotate around the screws in the coronal plane when compression is applied.
The present invention is directed at overcoming, or at least improving upon, the disadvantages of the prior art.